Size and Timing of Metamorphosis in Complex Life Cycles: Time Constraints and Variation

Abstract

Complex life cycles are characterized by niche shifts at the time of metamorphosis. Current models predict optimal sizes for metamorphosis based on maximizing growth, minimizing mortality, or some balance of these goals. These models predict optimal sizes that are independent of the time of metamorphosis. Reproduction and other major events in the life history of organisms are often constrained to seasons, and the state (e.g., mass) of the organism at that time is related to fitness. Therefore, an organism's state as well as the time that that state is achieved are central variables in these time—constrained life histories. We extend earlier theory to include explicit time constraints in three, hypothetical, complex life cycles. Dynamic optimization models are constructed to determine optimal time and mass trajectories for niche shifts. First, we consider the habitat shift at emergence in mayflies, where reproduction terminates a growth period in the first habitat and is constrained to a season. Second, we consider the habitat shift at metamorphosis in amphibians, where reproduction terminates a growth phase in the second habitat and reproduction is constrained to a single point in time. Third, we combine the first two effects to allow an extended period of reproduction in amphibians. Here optimal time and mass trajectories are determined for two niche shifts–the shift from aquatic to terrestrial habitat and the shift from a growth phase to a reproductive phase. We present analytical theory that allows both quantitative and qualitative predictions. Problem constructions and solutions are presented graphically to aid intuition in interpreting our results and extending the framework to other parameter values and other life—history examples. The general conclusion is that time constraints on complex life histories lead to optimal sizes for niche shifts that vary with time. In time—constrained life histories, any variation in the state of individuals at some time prior to reproduction will be preserved to some degree at reproduction. Therefore, in time—constrained life histories, we expect optimal switches in habitat use or life history stage to depend not only on state but also on the time that state is achieved.

Linking individuals with ecosystems: Experimentally identifying the relevant organizational scale for predicting trophic abundances

“…As a consequence, the effect of lower density of smaller grasshoppers appeared to be compensated by greater per capita foraging effort of the surviving individuals. Such sizebased differences in effort are expected whenever there is a high risk that small individuals with annual life cycles will fail to complete their development by the end of the season (10,22). Our results are consistent with theoretical predictions (9,11,12) that initially smaller individuals may feed more frequently than larger individuals to sustain higher growth rates despite the existence of some nonzero level of starvation or predation risk.…”

“…Furthermore, differences in initial size may influence the way that individuals tradeoff avoiding predators and foraging gains (development rate) (8). For instance, if the cost of avoiding predators in small individuals is a complete failure to mature by the end of the season, then initially smaller individuals may feed more frequently than larger individuals despite the existence of some nonzero level of predation risk (8)(9)(10)(11)(12). Over the course of a season, such size-dependent tradeoff behavior could result in differential direct effects of predators on the abundance of a given-sized prey, and indirect effects of the predators on the abundance of the resources used by prey of a given size.…”

“…All individuals must develop, mature, and reproduce within the course of 3 months. Individual grasshoppers that are initially smaller, and thus require longer time to mature, may have lower fitness than individuals who mature more rapidly because of an initial size advantage (9)(10)(11)(12). Furthermore, differences in initial size may influence the way that individuals tradeoff avoiding predators and foraging gains (development rate) (8).…”

Predators select against high growth rates and risk–taking behaviour in domestic trout populations

“…Gilliam & Fraser 1987;Anholt & Werner 1995 and in the field (Biro et al 2003a,b) support these predictions and show substantial increases in prey activity, use of risky habitats and greater predation mortality with declines in food abundance. Risk-taking behaviour probably results from constraints that force individuals to take risks, such as time constraints to reach a particular body state, or a baseline risk of predation that is independent of behaviour (Rowe & Ludwig 1991;Walters & Juanes 1993;Werner & Anholt 1993).…”

Life-history decisions under predation risk: Importance of a game perspective

“…Many animals exhibit ontogenetic shifts in food and habitat use as they increase in size (Rowe and Ludwig, 1991), and this is not limited to animals that undergo metamorphosis. Due to these shifts, reptiles, amphibians, ®sh and invertebrates are often exposed to di erent growth rates and di erent predation pressures during di erent stages of their development (reviewed in Werner and Gilliam, 1984).…”